Battery cell and lithium ion secondary battery

Abstract

The invention relates to the field of secondary batteries, and discloses a battery cell and a lithium ion secondary battery. A diaphragm in the battery cell comprises a base membrane, a functional coating is arranged on the surface of at least one side of the base membrane, the functional coating comprises N-H bonds, and the mole number alpha of the N-H bonds is larger than or equal to 10 mmol on the basis of the area of the functional coating. The functional coating comprising at least 10 mmol / m < 2 > of N-H bonds is arranged on the surface of the base membrane, so that the polarity of the diaphragm can be improved, and through the interaction between the N-H polar bonds and the electrolyte, the mass transfer process of the electrolyte at the diaphragm is promoted, the distribution of the electrolyte on the surface of a negative plate is optimized, and the lithium ion concentration difference of an interface is weakened in the battery charging process; the uniform deposition of lithium ions is promoted, and the growth of lithium dendrites is prevented, so that the cycle performance of the battery is improved, and the black spot lithium precipitation phenomenon is reduced.

Description

Technical Field

[0001] The present application relates to the field of secondary batteries, and in particular to a battery cell and a lithium-ion secondary battery. Background Art

[0002] During the use of lithium-ion secondary batteries, repeated charging and discharging consumes electrolyte. As the electrolyte is gradually consumed, the distribution of electrolyte in the center, corners, and near the tabs of the battery's negative electrode sheet becomes uneven, and even liquid-deficient areas may appear. Since there is no electrolyte in the liquid-deficient area to provide a transfer channel for lithium ions released from the positive electrode, the lithium ions cannot be embedded in the negative electrode active material in the corresponding area, resulting in the formation of black spots on the surface of the negative electrode sheet, which in turn causes battery capacity decay and even failure, reducing the battery cycle life. The precipitation of lithium from black spots also poses a safety risk. Summary of the Invention

[0003] In view of this, the technical problem to be solved by this application is to overcome the problem of poor battery cycle performance caused by uneven distribution of electrolyte in existing batteries and to alleviate the black spot lithium precipitation phenomenon at the negative electrode interface of lithium-ion batteries.

[0004] To achieve the above objectives, this application provides the following technical solutions:

[0005] According to an embodiment of the present application, in a first aspect, the present application provides a battery cell comprising a positive electrode sheet, a negative electrode sheet, a separator and an electrolyte, wherein the positive electrode sheet, the separator and the negative electrode sheet are stacked; the separator comprises a base film, and a functional coating is provided on at least one side of the base film; the functional coating comprises an NH bond; and the area of ​​the functional coating (unit: m 2 ) is used as the basis, the molar number of the NH bond α is ≥10 mmol.

[0006] In some optional embodiments, α further satisfies: α≤250 mmol.

[0007] In some optional embodiments, the functional coating includes at least one of melamine cyanurate, melamine polyphosphate, and ammonium polyphosphate.

[0008] In some optional embodiments, the electrolyte includes a nitrile additive. Based on the mass of the electrolyte, the content of the nitrile additive is β%, and α and β satisfy: α>1.7*β.

[0009] In some optional embodiments, β% satisfies: 0<β%≤10%.

[0010] In some optional embodiments, the nitrile additives include acetonitrile, malononitrile, succinonitrile, glutaronitrile, adiponitrile, 1,3,6-hexanetrinitrile, 1,3,5-pentanetrinitrile, ethylene glycol bispropionitrile ether, hexafluorocyclotriphosphazene, pentafluoroethoxycyclotriphosphazene, pentafluorophenoxycyclotriphosphazene, 1,4-dicyano-2-butene, p-fluorobenzonitrile, p-methylbenzonitrile, 2-fluoroadiponitrile, 2,2-difluorosuccinonitrile, tricyanobenzene, acrylonitrile, crotononitrile, trans-butenedicononitrile, trans-hexenedicononitrile, 3-(trimethylsilyloxy)propionitrile, 1,3,4,6-hexanetetranitrile, 1,2,4,5-benzenetetracarbonitrile, 2,3,5,6-pyrazinetetranitrile, sebacononitrile, azelaic acid dicyanobenzene, pyridine at least one of -3,4-dinitrile, 2,5-dicyanopyridine, 2,2,3,3-tetrafluorosuccinonitrile, tetrafluoroterephthalonitrile, 4-tetrahydrothiopyrylidenemalononitrile, 1,4,5,6-tetrahydro-5,6-dioxo-2,3-pyrazinedicarbonitrile, 1,3,5-cyclohexanetricarbonitrile, 1,2,3-propanetricarbonitrile, glyceroltricarbonitrile, tris(3-cyanopropyl)phosphate, 1,1,3,3-propanetetracarbonitrile, 1,2,2,3-tetracyanopropane, 3-methyl-3-propyl-cyclopropane-1,1,2,2-tetracarbonitrile, 7,7,8,8-tetracyanoquinodimethane, tetracyanoethylene, di(cyanoethoxy)ethane, tris(cyanoethoxy)propane, and tetra(cyanoethoxy)butane.

[0011] In some optional embodiments, the particle diameter D of the melamine cyanurate or its derivative is 0.1 μm-5 μm.

[0012] In some optional embodiments, the functional coating further includes at least one of a dispersant and a first binder.

[0013] In some optional embodiments, the dispersant includes at least one of polyvinyl pyrrolidone, polyacrylate, modified styrene-maleic acid copolymer, styrene-maleic anhydride ester, polyacrylate ammonium salt, polyacrylate sodium salt, polyether phosphate, and silicone-modified polyether phosphate.

[0014] In some optional embodiments, the first binder includes at least one of polyacrylic acid, polyacrylonitrile, polyacrylate, polystyrene-methyl acrylate, styrene-methyl methacrylate-acrylonitrile copolymer, isooctyl acrylate-styrene-acrylonitrile copolymer, isooctyl acrylate-styrene-methyl methacrylate, styrene-methacrylic acid-acrylonitrile copolymer, isooctyl acrylate-styrene-methacrylic acid, polyvinylidene fluoride, vinylidene fluoride-hexafluoropropylene copolymer, polytetrafluoroethylene, polyetherimide, and polyimide.

[0015] In some optional embodiments, the porosity of the base film is 28%-60%, preferably 32%-40%.

[0016] In some optional embodiments, the puncture strength of the basement membrane is greater than 200 gf.

[0017] In some optional embodiments, the base film has a thickness of 3 μm-20 μm.

[0018] In some optional embodiments, the base film includes a laminate of one or more of polyethylene, polypropylene, polyimide, polyethylene terephthalate, 1313 aramid, and 1414 aramid.

[0019] In some optional embodiments, the negative electrode sheet includes a negative electrode current collector and a negative electrode active layer arranged on at least one side surface of the negative electrode current collector in the thickness direction, the negative electrode active layer includes a negative electrode active material, a conductive agent and a second binder; the negative electrode active material includes at least one of natural graphite, artificial graphite, silicon carbon, and silicon oxide.

[0020] In some optional embodiments, based on the mass of the negative electrode active layer, the content of the negative electrode active material is ≥92%.

[0021] According to an embodiment of the present application, in a second aspect, the present application provides a lithium-ion secondary battery comprising the battery cell described in the first aspect.

[0022] Compared with the prior art, this application has the following beneficial effects:

[0023] The separator in the battery cell provided in the present application comprises a base film, and a functional coating is provided on at least one side surface of the base film, wherein the functional coating comprises an NH bond, and the area of ​​the functional coating (unit: m 2 ) is the basis of the number of moles of NH bonds α≥10mmol. The present application is provided with at least 10mmol / m 2 The functional coating of the NH bond, since the NH bond is a polar bond, can increase the polarity of the diaphragm, so that when the diaphragm contacts the electrolyte, through the interaction between these NH polar bonds and the electrolyte, during the battery charging process, it can promote the mass transfer process of the electrolyte at the diaphragm, optimize the distribution of the electrolyte on the surface of the negative electrode, weaken the interface lithium ion concentration difference, promote the uniform deposition of lithium ions, prevent the growth of lithium dendrites, thereby improving the cycle performance of the battery and weakening the black spot lithium precipitation phenomenon at the negative electrode interface.

[0024] Additional aspects and advantages of the embodiments of the present application will be described and shown in part in the subsequent description, or explained through the implementation of the embodiments of the present application. DETAILED DESCRIPTION

[0025] The following examples are provided to further better understand the present application, but are not limited to the best implementation mode described herein, and do not limit the content and protection scope of the present application. Any product identical or similar to the present application obtained by anyone under the inspiration of the present application or by combining the features of the present application with other prior arts shall fall within the scope of protection of the present application.

[0026] In order to solve the problem of poor battery cycle performance of lithium-ion secondary batteries due to uneven distribution of electrolyte in related technologies and to reduce the phenomenon of black spots and lithium precipitation at the negative electrode, this application proposes the following solution.

[0027] According to an embodiment of the present application, in a first aspect, a battery cell is provided, comprising a positive electrode sheet, a negative electrode sheet, a separator and an electrolyte, wherein the positive electrode sheet, the separator and the negative electrode sheet are stacked; the separator comprises a base film, and a functional coating is provided on at least one side surface of the base film; the functional coating comprises an NH bond; and the area of ​​the functional coating (unit: m 2 ) is used as the basis, the molar number of the NH bond α is ≥10 mmol.

[0028] The present application has found that the graphite negative electrode active material has poor polarity and insufficient wettability with the electrolyte. It is greatly affected by the mass transfer of the electrolyte during the electrochemical reaction, and is prone to poor battery cycle performance and interface problems such as black spot lithium precipitation. In view of this, the present application provides a functional coating including NH bonds on at least one side of the base film of the diaphragm. Since the NH bonds are polar bonds, the polarity of the diaphragm can be increased, so that when the diaphragm is in contact with the electrolyte, the interaction between these NH polar bonds and the electrolyte can promote the mass transfer process of the electrolyte at the diaphragm, improve the distribution of the electrolyte on the surface of the negative electrode during charging, weaken the difference in lithium ion concentration at the interface, promote the uniform deposition of lithium ions, prevent the growth of lithium dendrites, thereby improving the cycle performance of the battery and reducing the black spot lithium precipitation phenomenon at the negative electrode interface. In addition, the content of NH bonds in the functional coating cannot be too low. The area of ​​the functional coating (unit: m 2 ) is used as the basis, and the molar number of the NH bond α is ≥10 mmol, otherwise the above effect cannot be achieved.

[0029] It should be noted that the molar number α of the NH bonds in the functional coating is determined by the following method:

[0030] Using N-methylpyrrolidone (NMP), the area of ​​the membrane is S (unit: m 2) is cleaned and dried, and the coating is scraped off to obtain a coating material with a weight of G, which is ground and mixed with potassium bromide powder to form a tablet with a total mass of 150 mg. The first functional group peak intensity I is obtained by infrared spectrometer (FTIR) testing; an external standard sample with a weight gradient of 0.1*G, G, 5*G, and 10*G is divided by the molecular weight M to obtain the corresponding molar weight, and a total of 150 mg of the external standard sample and potassium bromide powder are made into a tablet, and the first functional group peak intensity is measured by FTIR. A linear relationship is fitted between the molar weight of the external standard sample and the peak intensity of the first functional group; the I value is substituted into the fitted relationship to obtain the molar weight Mg of the first functional group in the coating with a weight of G, and then the molar number of NH bonds in the functional coating is calculated according to α=Mg / S.

[0031] It is worth mentioning that the NH bond in the functional coating comes from at least one of melamine cyanurate, melamine polyphosphate, and ammonium polyphosphate. Melamine cyanurate (MCA for short) is a supramolecular material formed by self-assembly of melamine and cyanuric acid through hydrogen bonds. Compared with other NH bond donors, each molecular unit of MCA contains multiple NH bonds, which can provide a large number of hydrogen bond donor sites to form a hydrogen bond network. The dense hydrogen bond network gives the material excellent thermal stability and mechanical strength, and MCA mainly generates carbon dioxide, nitrogen and water after decomposition, which is pollution-free to the environment. Therefore, applying MCA to the diaphragm of the present application can increase the polarity of the diaphragm with a smaller addition amount, promote the mass transfer of the electrolyte at the diaphragm, avoid excessive addition of MCA, and cause the diaphragm to be too thick and the air permeability to deteriorate, thereby affecting the volume energy density and cycle life of the battery, which is not conducive to the improvement of the problem of black spot lithium precipitation.

[0032] In some embodiments, the molar number α of the NH bond also satisfies: α≤250mmol. The present application study found that if the content of NH bonds in the functional coating is too high, that is, α>250mmol, it means that the amount of melamine cyanurate or its derivatives added to the diaphragm is too much, which will lead to a larger diaphragm thickness and poorer air permeability, which is not conducive to the improvement of the battery's volume energy density, cycle performance and black spot lithium precipitation problem. Therefore, controlling the molar number α of the NH bond in the functional coating between 10-250mmol can ensure that the battery has a higher volume energy density, better cycle performance, and reduces the black spot lithium precipitation phenomenon.

[0033] Illustratively, the molar number α of NH bonds in the functional coating can be, for example, 10 mmol, 25 mmol, 50 mmol, 75 mmol, 100 mmol, 125 mmol, 150 mmol, 175 mmol, 200 mmol, 225 mmol, 250 mmol, etc., or a value within the range formed by any two of the above values.

[0034] In some embodiments, the electrolyte includes nitrile additives, which have good antioxidant properties and can protect the positive electrode structure. However, nitrile additives are not friendly to the negative electrode and will cause the negative electrode interface polarization impedance to increase, hindering the negative electrode from inserting lithium.

[0035] Based on this, the present application satisfies the following conditions by regulating the content β% of nitrile additives in the electrolyte and the molar number α of NH bonds in the functional coating of the diaphragm: α>1.7*β, α and β are positively correlated. The higher the content of nitrile additives in the electrolyte, the greater the difficulty of lithium insertion in the negative electrode. At this time, it is necessary to simultaneously increase the content of NH bonds in the functional coating of the diaphragm to further promote the mass transfer of the electrolyte at the diaphragm during charging, reduce the difficulty of lithium insertion in the negative electrode, prevent lithium plating at the negative electrode, and thus improve the cycle performance of the battery and reduce the phenomenon of black spot lithium plating.

[0036] Specifically, in some embodiments, the mass content β% of the nitrile additive in the electrolyte satisfies the following conditions: 0<β%≤10%. This, on the one hand, increases the voltage platform of the positive electrode active material, allowing the high-voltage positive electrode to ensure high energy density of the battery; on the other hand, it prevents excessive nitrile additive content from increasing battery impedance, which is detrimental to cycle and rate performance. Thirdly, the synergistic effect between β and the molar number α of NH bonds in the functional coating of the separator can reduce the difficulty of lithium insertion in the negative electrode, prevent lithium deposition at the negative electrode, and thus improve the battery's cycle performance and reduce the phenomenon of black spot lithium deposition.

[0037] Exemplarily, the mass content β% of the nitrile additive in the electrolyte can be, for example, 0.1%, 0.5%, 1%, 1.5%, 2%, 2.5%, 3%, 3.5%, 4%, 4.5%, 5%, 5.5%, 6%, 6.5%, 7%, 7.5%, 8%, 8.5%, 9%, 9.5%, 10%, etc., or a value within the range of any two of the above values.

[0038] As an example, the nitrile additives include acetonitrile, malononitrile, succinonitrile, glutaronitrile, adiponitrile, 1,3,6-hexanetrinitrile, 1,3,5-pentanetrinitrile, ethylene glycol bispropionitrile ether, hexafluorocyclotriphosphazene, pentafluoroethoxycyclotriphosphazene, pentafluorophenoxycyclotriphosphazene, 1,4-dicyano-2-butene, p-fluorobenzonitrile, p-methylbenzonitrile, 2-fluoroadiponitrile, 2,2-difluorosuccinonitrile, tricyanobenzene, acrylonitrile, crotononitrile, trans-butenedicononitrile, trans-hexenedicononitrile, 3-(trimethylsilyloxy)propionitrile, 1,3,4,6-hexanetetranitrile, 1,2,4,5-benzenetetracarbonitrile, 2,3,5,6-pyrazinetetranitrile, sebacononitrile, azelaic acid dicyanobenzene, dicyanobenzene, pyridine-3, At least one of 4-dicarbonitrile, 2,5-dicyanopyridine, 2,2,3,3-tetrafluorosuccinonitrile, tetrafluoroterephthalonitrile, 4-tetrahydrothiopyrylamide malononitrile, 1,4,5,6-tetrahydro-5,6-dioxo-2,3-pyrazine dicarbonitrile, 1,3,5-cyclohexane tricarbonitrile, 1,2,3-propane tricarbonitrile, glycerol tricarbonitrile, tris(3-cyanopropyl)phosphate, 1,1,3,3-propane tetracarbonitrile, 1,2,2,3-tetracyanopropane, 3-methyl-3-propyl-cyclopropane-1,1,2,2-tetracarbonitrile, 7,7,8,8-tetracyanoquinodimethane, tetracyanoethylene, bis(cyanoethoxy)ethane, tris(cyanoethoxy)propane, and tetra(cyanoethoxy)butane.

[0039] It should be noted that when the molar number α of the NH bond in the functional coating is in the range of 10-250 mmol, the thickness d of the functional coating is correspondingly between 0.5-10 μm. In this way, on the one hand, the polarity of the diaphragm can be improved, the mass transfer of the electrolyte in the diaphragm can be promoted, and lithium plating of the negative electrode can be prevented. On the other hand, it can also avoid the diaphragm being too thick and the permeability being poor, which is not conducive to the volume energy density, cycle performance and safety of the battery.

[0040] Exemplarily, the thickness d of the functional coating can be 0.5 μm, 1 μm, 1.5 μm, 2 μm, 2.5 μm, 3 μm, 3.5 μm, 4 μm, 4.5 μm, 5 μm, 5.5 μm, 6 μm, 6.5 μm, 7 μm, 7.5 μm, 8 μm, 8.5 μm, 9 μm, 9.5 μm, 10 μm, etc., or a value within the range of any two of the above values.

[0041] It is understandable that the thickness of the functional coating is closely related to the particle diameter of melamine cyanurate or its derivatives, and the particle diameter of melamine cyanurate or its derivatives determines the minimum thickness of the functional coating. In some embodiments, the particle diameter D of melamine cyanurate or its derivatives is 0.1 μm-5 μm, thereby ensuring that the functional coating has an appropriate thickness, which not only improves the wetting effect of the electrolyte on the diaphragm and prevents lithium precipitation at the negative electrode, but also avoids the diaphragm thickness being too large and the permeability being deteriorated. If the particle diameter of melamine cyanurate or its derivatives is too large, the functional coating will be too thick, thereby affecting the permeability of the diaphragm, which is not conducive to the volume energy density, cycle performance and safety of the battery.

[0042] Illustratively, the particle diameter D of the melamine cyanurate or its derivative is 0.1 μm, 0.5 μm, 1 μm, 1.5 μm, 2 μm, 2.5 μm, 3 μm, 3.5 μm, 4 μm, 4.5 μm, 5 μm, etc., or a value within the range of any two of the above values.

[0043] It should be noted that the test method for the particle diameter D of melamine cyanurate or its derivatives includes: using a field emission electron microscope to take photos at 25,000 times, arbitrarily selecting 200 particles from the photos, recording the long side dimension of these particle images as Ll and the short side dimension as Ls, and calculating the D value according to (Ll+Ls) / 2, in units of μm.

[0044] In some embodiments, the functional coating is obtained by applying a slurry comprising an NH bond donor (i.e., melamine cyanurate or a derivative thereof) to at least one surface of the base film in the thickness direction. Specifically, the process comprises the following steps:

[0045] Under high-speed stirring, deionized water and melamine cyanurate or its derivatives are mixed in a certain proportion, and stirring is continued to prevent powder agglomeration. An appropriate amount of dispersant, a first binder and deionized water are added and stirred to obtain a coating slurry with a solid content of 17-40% and a viscosity of 15.3-64.5 mPa·s. The coating slurry is evenly supplied to the coating head through a pumping system, and the base film is sent to the coating area for coating, and then baked to obtain a functional coating with an area density of 1.5-4.8 g / m 2 .

[0046] It is understandable that adding a dispersant during the preparation of the functional coating can promote the uniform dispersion of melamine cyanurate or its derivatives in the slurry, ensure the uniformity of the subsequent slurry coating, and then ensure the uniform distribution of the electrode liquid on the surface of the negative electrode, prevent lithium plating on the negative electrode, and improve the battery cycle performance and safety.

[0047] As an example, the dispersant includes at least one of polyvinyl pyrrolidone (PVP), polyacrylate, modified styrene-maleic acid copolymer, styrene-maleic anhydride ester, polyacrylate ammonium salt, polyacrylate sodium salt, polyether phosphate, and silicone-modified polyether phosphate.

[0048] It can be understood that adding the first binder during the preparation of the functional coating not only improves the film-forming property and mechanical strength of the functional coating, but also enables the functional coating to be more tightly bonded to the surface of the base film, preventing the functional coating from falling off during the battery cycle, thereby ensuring the battery's cycle performance and safety.

[0049] As an example, the first binder includes at least one of polyacrylic acid (PAA), polyacrylonitrile, polyacrylate, polystyrene-methyl acrylate, styrene-methyl methacrylate-acrylonitrile copolymer, isooctyl acrylate-styrene-acrylonitrile copolymer, isooctyl acrylate-styrene-methyl methacrylate, styrene-methacrylic acid-acrylonitrile copolymer, isooctyl acrylate-styrene-methacrylic acid, polyvinylidene fluoride, vinylidene fluoride-hexafluoropropylene copolymer, polytetrafluoroethylene, polyetherimide, and polyimide.

[0050] In some embodiments, the porosity of the base membrane is 30%-60%, preferably 35%-45%. On the one hand, the porosity of the base membrane determines the number of migration channels for lithium ions in the diaphragm. The use of a base membrane with the above porosity can ensure that the diaphragm provides more lithium ion transmission paths, reduces the internal resistance of the battery, improves the charge and discharge efficiency, delays capacity decay, and extends the cycle life; on the other hand, it can ensure that the diaphragm has good tensile strength and puncture strength, reduces the risk of lithium dendrites piercing the diaphragm, and improves the high-temperature safety performance of the battery. When the porosity of the base membrane is too low, ion transmission is hindered, affecting the rate performance and cycle performance of the battery; when the porosity of the base membrane is too high, the strength of the diaphragm deteriorates, which is not conducive to the high-temperature safety performance of the battery.

[0051] The porosity of the base film is measured and calculated using a true density meter. For example, the porosity of the base film can be 28%, 32%, 40%, 45%, 50%, 55%, 60%, or a value within a range formed by any two of the above values.

[0052] In some embodiments, the base film has a puncture strength greater than 200gf. Puncture strength is a key indicator of a battery separator's ability to resist puncture by sharp objects (such as lithium dendrites or metal particles). Using a base film with a puncture strength greater than 200gf to produce a battery separator can significantly reduce the risk of lithium dendrites or residual metal particles such as copper and aluminum debris from the battery manufacturing process piercing the separator, thereby improving battery safety. When the puncture strength of the base film is too low, the battery cell self-discharge is high, resulting in a shortened battery life and increased safety risks. When the puncture strength of the base film is too high, the efficiency of the separator in transporting lithium ions is affected, affecting the battery's rate and cycle performance.

[0053] The puncture strength of the base film is tested in accordance with GB / T36363-2018, "Polyolefin Separators for Lithium-ion Batteries." For example, the puncture strength of the base film can be 210 gf, 250 gf, 300 gf, 350 gf, 400 gf, 450 gf, 500 gf, 550 gf, 600 gf, 650 gf, or a value within a range between any two of the aforementioned values.

[0054] In some embodiments, the base film has a thickness of 3 μm to 20 μm. The thickness of the base film directly affects the thickness of the separator. Using a base film of this thickness to manufacture the separator ensures that the separator has good mechanical strength and lithium ion transmission rate, thereby ensuring that the battery has high volumetric energy density, good rate and cycle performance, and good high-temperature safety.

[0055] The thickness of the base film is tested in accordance with the provisions of GB / T36363-2018 "Polyolefin Separators for Lithium-ion Batteries." For example, the thickness of the base film can be 3 μm, 5 μm, 7.5 μm, 10 μm, 12.5 μm, 15 μm, 17.5 μm, 20 μm, or a value within a range formed by any two of the aforementioned values.

[0056] Exemplarily, the base film comprises a laminate of one or more of polyethylene, polypropylene, polyimide, polyethylene terephthalate, 1313 aramid, and 1414 aramid. A laminate is a multilayer film formed by sequentially laminating and hot-rolling multiple polymer films of the same or different materials.

[0057] In some embodiments, the negative electrode sheet includes a negative electrode current collector and a negative electrode active layer disposed on at least one surface of the negative electrode current collector in a thickness direction, wherein the negative electrode active layer includes a negative electrode active material, a conductive agent, and a second binder.

[0058] As an example, the negative electrode active material includes at least one of natural graphite, artificial graphite, silicon carbon, and silicon oxide.

[0059] As an example, the conductive agent includes at least one of conductive carbon black (SP), activated carbon, acetylene black, Ketjen black, conductive graphite, conductive carbon fiber, carbon nanotubes, metal powder, carbon fiber, and graphene, and the metal powder includes at least one of magnesium powder, copper powder, zirconium powder, lithium powder, calcium powder, manganese powder, sodium powder, and aluminum powder.

[0060] As an example, the second binder includes polyacrylic acid (PAA), polyacrylate, styrene-butadiene rubber (SBR), carboxymethyl cellulose (CMC), polyacrylonitrile (PAN), polyvinylidene fluoride (PVDF), polyvinyl alcohol (PVA), polytetrafluoroethylene (PTFE), polyolefins (such as polypropylene PP, polyethylene PE and other olefin copolymers), fluorinated rubber, polyimide (PI), polyvinylidene fluoride-hexafluoropropylene copolymer (PVDF-HFP) or at least one of their derivatives.

[0061] In some embodiments, based on the mass of the negative electrode active layer, the content of the negative electrode active material is ≥92%. This ensures that the negative electrode active layer has a higher surface density, thereby helping to improve the energy density of the battery.

[0062] As an example, the content of the negative electrode active material in the negative electrode active layer can be 92%, 93%, 94%, 95%, 96%, 97%, 98%, 99%, etc., or a value within the range formed by any two of the above values.

[0063] According to an embodiment of the present application, in a second aspect, a lithium-ion secondary battery is provided, comprising the battery cell described in the first aspect. The lithium-ion secondary battery assembled using the battery cell has the advantages of good cycle performance and excellent safety.

[0064] The present application is further described in detail below with reference to specific examples. These examples are not to be construed as limiting the scope of protection claimed in this application. Where specific experimental procedures or conditions are not specified in the Examples and Comparative Examples, the procedures or conditions of conventional experimental procedures described in the literature in this field can be followed. Where the manufacturer of the reagents or instruments used is not specified, they are all commercially available conventional reagent products.

[0065] Example 1

[0066] This embodiment provides a method for preparing a lithium-ion secondary battery, comprising the following steps:

[0067] (1) Preparation of positive electrode sheet

[0068] First, lithium cobalt oxide is mixed with a conductive agent (the mass ratio of SP to activated carbon is 1:1), PVDF is added, and the mixture is dissolved in NMP solvent at a mass ratio of lithium cobalt oxide to conductive agent and PVDF of 97.5:1.35:1.15 to ensure uniform mixing to obtain a positive electrode slurry; then, the positive electrode slurry is evenly coated on both sides of the aluminum foil current collector in the thickness direction and dried to form a positive electrode membrane; finally, the positive electrode membrane is cold pressed, cut into sheets, and the electrode ears are welded to obtain a positive electrode sheet.

[0069] (2) Preparation of negative electrode sheet

[0070] First, graphite, SP, CMC and SBR are mixed in a mass ratio of 97.2:0.6:1.0:1.2, dissolved in deionized water, and stirred thoroughly to form a negative electrode slurry; then, the negative electrode slurry is evenly coated on both sides of the copper foil current collector in the thickness direction and dried to form a negative electrode membrane; finally, the negative electrode membrane is cold pressed, cut into sheets, and electrode ears are welded to obtain a negative electrode sheet.

[0071] (3) Preparation of electrolyte

[0072] Ethylene carbonate, diethyl carbonate, methyl ethyl carbonate, and vinylene carbonate are mixed in a mass ratio of 8:85:5:2 to obtain a mixed solvent. Lithium salt LiPF6 and a nitrile additive 1,3,6-hexanetrinitrile are added to the mixed solvent and mixed to obtain an electrolyte. The mass ratio of lithium salt to mixed solvent is 8:92, and the content of the nitrile additive is 5%, based on the total mass of the lithium salt and the mixed solvent.

[0073] (4) Diaphragm preparation

[0074] a. Soak the polyethylene film in anhydrous ethanol for 50s-70s, remove and drain, repeat three times, place in an oven, control the oven temperature to 45 ° C, and dry for 3h;

[0075] b. Under high-speed stirring conditions, deionized water and melamine cyanurate were mixed in a mass ratio of 75:25, and then 0.5w% PVP and 5w% PAA and an appropriate amount of deionized water were added, and stirring was continued to prevent the powder from agglomerating to obtain a slurry A with a solid content of 17%;

[0076] c. The polyethylene film treated in step a is fed into the coating area at a speed of 2 m / min, and the slurry A is evenly supplied to the slit coating head through the pumping system. After single-sided coating and baking, the coating surface density is 1.5 g / m 2 Coating film B.

[0077] d. The binder Yindi Le FS505A was mixed with deionized water to obtain a slurry C having a solid content of 6%;

[0078] e. The coating film B is fed into the coating area at a speed of 2m / min. The slurry C is evenly supplied to the slit coating head through the pumping system. After double-sided coating and 60°C hot air drying, the diaphragm is obtained. The single-sided density of the adhesive layer is 0.15g / m 2 .

[0079] (5) Preparation of lithium-ion secondary batteries

[0080] The prepared positive electrode sheet, separator and negative electrode sheet are wound to prepare a bare battery cell, wherein the side of the separator coated with melamine cyanurate faces the positive electrode sheet; after drying, liquid injection and packaging, a lithium-ion secondary battery is obtained.

[0081] Example 2

[0082] The preparation method of Example 2 is basically the same as that of Example 1, except that in step (5), the side of the diaphragm coated with melamine cyanurate faces the negative electrode sheet.

[0083] Example 3

[0084] The preparation method of Example 3 is basically the same as that of Example 1, except that the coating surface density is set to 0.75 g / m in step (4) c. 2 After one-side coating and baking, the other side of the base film is coated again, and the single-side coating surface density is controlled to be 0.75m 2 , after baking, a coating film B is obtained. Correspondingly, both surfaces of the base film of the diaphragm in step (5) are coated with melamine cyanurate, facing the positive and negative electrode sheets respectively.

[0085] The preparation methods of Examples 4 to 27 and Comparative Examples 1 to 2 are basically the same as those of Example 1, with the differences shown in Table 1. In Comparative Example 2, aluminum oxide was used instead of melamine cyanurate in the diaphragm preparation step.

[0086] Table 1

[0087]

[0088]

[0089] Test Case

[0090] 1. 25°C Cycling Performance Test: At 25°C ± 2°C, charge the lithium-ion secondary battery at a constant current and constant voltage (CCV) of 0.7C to 4.5V, cut off at 0.05C, then discharge at a constant current of 0.2C to 3.0V. The initial discharge capacity is recorded as C0. Let it rest for 10 minutes. The cycling protocol is as follows: charge at 2C to 4.25V, switch to a constant voltage of 1C, charge at 1C to 4.5V (full-charge cutoff voltage), and then cut off at a constant voltage of 0.05C; discharge at 0.5C to 3.0V. After 800 cycles, charge at a constant current and constant voltage of 0.5C to 4.5V, cut off at 0.05C, and then discharge at a constant current of 0.2C to 3.0V. The discharge capacity is recorded as C1. Calculate the capacity retention rate: C = C1 / C0 * 100%.

[0091] Subsequently, charge at 0.5C to 4.5V (full charge cut-off voltage) and maintain constant voltage to 0.05C cut-off, maintain the battery voltage at 4.5±0.05V, use scissors or other tools to remove the outer packaging film and the tabs of the battery, peel off the diaphragm, positive electrode, and negative electrode stack to obtain the negative electrode sheet, observe the color of the negative electrode sheet, the golden yellow area is the normal area, the black area is the black spot area without lithium embedding, and the gray or grayish white area is the lithium deposition location.

[0092] 2. 45°C Cycling Performance Test: At 25°C±2°C, charge the lithium-ion secondary battery at a constant current and constant voltage (CCV) of 0.7C to 4.5V, cut off at 0.05C, then discharge at a constant current of 0.2C to 3.0V. The initial discharge capacity is recorded as C0. Let it rest for 10 minutes. The cycling protocol is as follows: charge at 2C to 4.25V, switch to a constant voltage of 1C, charge at 1C to 4.5V (full-charge cutoff voltage), and then cut off at a constant voltage of 0.05C; discharge at 0.5C to 3.0V. After 600 cycles, charge at a constant current and constant voltage of 0.5C to 4.5V, cut off at 0.05C, and then discharge at a constant current of 0.2C to 3.0V. The discharge capacity is recorded as C1. Calculate the capacity retention rate: C = C1 / C0 * 100%.

[0093] Subsequently, charge at 0.5C to 4.5V (full charge cut-off voltage) and maintain constant voltage to 0.05C cut-off, maintain the battery voltage at 4.5±0.05V, use scissors or other tools to remove the outer packaging film and the tabs of the battery, peel off the diaphragm, positive electrode, and negative electrode stack to obtain the negative electrode sheet, observe the color of the negative electrode sheet, the golden yellow area is the normal area, the black area is the black spot area without lithium embedding, and the gray or grayish white area is the lithium deposition location.

[0094] 3. Oven Temperature Test: In a 25±3°C environment, charge the battery at 0.5°C to a maximum voltage of 4.5V, cut off at 0.05°C, and let it rest for 10 minutes. Place the battery in a 130°C thermal shock test chamber, raise the temperature to 132°C at a rate of 5°C±2°C / min, and hold for 63 minutes. Observe for fires and record the data, expressing the number of batteries that caught fire as the ratio of the number of batteries tested to the number of batteries that caught fire.

[0095] 4. Volumetric Energy Density Test: At 25±3°C, fully charge at 0.5C to a maximum voltage of 4.5V, cut off at 0.05C, let stand for 10 minutes, and discharge at 0.5C to a minimum voltage of 3.0V, cut off at 0.05C. Divide the battery's discharge energy by its length, width, and thickness to obtain the battery's energy density (ED) in wh / L.

[0096] 5. Rate test: fully charge at 0.5C in a constant temperature room at 25±3℃, cut-off current at 0.05C, let it stand for 10 minutes, then discharge at a rate of 0.2C / 2.5C to a cut-off voltage of 3.0V in a constant temperature room or incubator environment;

[0097] 2.5C rate capacity retention rate = (2.5C discharge capacity / 0.2C discharge capacity)*100%.

[0098] The above test results are shown in Table 2, where ○ indicates that there is no black spot lithium deposition, and the interface between the black spots and the lithium deposition does not exceed 0.5% of the total area of ​​the electrode; ▲ indicates slight black spot lithium deposition, and the interface between the black spots and the lithium deposition does not exceed 5% of the total area of ​​the electrode; ● indicates severe black spot lithium deposition, and the interface between the black spots and the lithium deposition exceeds 10% of the total area of ​​the electrode.

[0099] Table 2

[0100]

[0101]

[0102] It can be seen from Tables 1 and 2 that compared with Comparative Examples 1-2, the NH bond content in the functional coating of the diaphragm of Examples 1-6 is ≥10 mmol, which can significantly improve the cycle capacity retention rate of the battery. This shows that introducing an appropriate amount of NH bonds in the functional coating of the diaphragm can significantly improve the negative electrode lithium plating.

[0103] Compared with Example 1, the content of nitrile additives in the electrolyte of Example 9 is too high, which affects the cycle and rate performance of the battery; the content of nitrile additives in the electrolyte of Example 10 is relatively high, but the NH bond content in the functional coating of the diaphragm is relatively low, which does not meet α>1.7*β, resulting in a significant decrease in the cycle and rate performance of the battery; and the porosity of the base film in the diaphragm of Examples 21-22 is relatively small, which affects the transmission of lithium ions, resulting in a decrease in the cycle and rate performance of the battery.

[0104] Obviously, the above embodiments are merely examples for clarity of explanation and are not intended to limit the implementation methods. Those skilled in the art will appreciate that other variations or modifications can be made based on the above description. It is not necessary and impossible to enumerate all implementation methods here. Obvious variations or modifications arising therefrom remain within the scope of protection of this application.

Claims

1. A battery cell comprising a positive electrode sheet, a negative electrode sheet, a separator and an electrolyte, wherein the positive electrode sheet, the separator and the negative electrode sheet are stacked; characterized in that: The diaphragm includes a base film, and a functional coating is provided on at least one side surface of the base film; The functional coating comprises NH bonds, and the area of ​​the functional coating (unit: m 2 ) is used as the basis, the molar number of the NH bond α is ≥10 mmol.

2. The battery cell according to claim 1, characterized in that α also satisfies: α≤250mmol; And / or, the functional coating comprises at least one of melamine cyanurate, melamine polyphosphate, and ammonium polyphosphate.

3. The battery cell according to claim 1 or 2, characterized in that: The electrolyte includes a nitrile additive. Based on the mass of the electrolyte, the content of the nitrile additive is β%, and α and β satisfy: α>1.7*β.

4. The battery cell according to claim 3, characterized in that β% satisfies: 0<β≤10; and / or, the nitrile additives include acetonitrile, malononitrile, succinonitrile, glutaronitrile, adiponitrile, 1,3,6-hexanetrinitrile, 1,3,5-pentanetrinitrile, ethylene glycol bispropionitrile ether, hexafluorocyclotriphosphazene, pentafluoroethoxycyclotriphosphazene, pentafluorophenoxycyclotriphosphazene, 1,4-dicyano-2-butene, p-fluorobenzonitrile, p-methylbenzonitrile, 2-fluoroadiponitrile, 2,2-difluorosuccinonitrile, tricyanobenzene, acrylonitrile, crotononitrile, trans-butenedicononitrile, trans-hexenedicononitrile, 3-(trimethylsilyloxy)propionitrile, 1,3,4,6-hexanetetranitrile, 1,2,4,5-benzenetetracarbonitrile, 2,3,5,6-pyrazinetetranitrile, sebacononitrile, azelaic acid dicyanobenzene, pyridine-3,4 -dinitrile, 2,5-dicyanopyridine, 2,2,3,3-tetrafluorosuccinonitrile, tetrafluoroterephthalonitrile, 4-tetrahydrothiopyrylamide malononitrile, 1,4,5,6-tetrahydro-5,6-dioxo-2,3-pyrazine dicarbonitrile, 1,3,5-cyclohexane tricarbonitrile, 1,2,3-propane tricarbonitrile, glycerol tricarbonitrile, tris(3-cyanopropyl)phosphate, 1,1,3,3-propane tetracarbonitrile, 1,2,2,3-tetracyanopropane, 3-methyl-3-propyl-cyclopropane-1,1,2,2-tetracarbonitrile, 7,7,8,8-tetracyanoquinodimethane, tetracyanoethylene, di(cyanoethoxy)ethane, tris(cyanoethoxy)propane, tetra(cyanoethoxy)butane.

5. The battery cell according to claim 1 or 2, characterized in that: The particle diameter D of the melamine cyanurate or its derivative is 0.1 μm-5 μm; And / or, the functional coating further includes at least one of a dispersant and a first binder.

6. The battery cell according to claim 5, characterized in that The dispersant comprises at least one of polyvinyl pyrrolidone, polyacrylate, modified styrene-maleic acid copolymer, styrene-maleic anhydride ester, polyacrylate ammonium salt, polyacrylate sodium salt, polyether phosphate, and organosilicon-modified polyether phosphate; And / or, the first binder includes at least one of polyacrylic acid, polyacrylonitrile, polyacrylate, polystyrene-methyl acrylate, styrene-methyl methacrylate-acrylonitrile copolymer, isooctyl acrylate-styrene-acrylonitrile copolymer, isooctyl acrylate-styrene-methyl methacrylate, styrene-methacrylic acid-acrylonitrile copolymer, isooctyl acrylate-styrene-methacrylic acid, polyvinylidene fluoride, vinylidene fluoride-hexafluoropropylene copolymer, polytetrafluoroethylene, polyetherimide, and polyimide.

7. The battery cell according to claim 1, characterized in that The porosity of the base film is 28%-60%, preferably 32%-40%; and / or, the basement membrane has a puncture strength greater than 200 gf; And / or, the base film has a thickness of 3 μm-20 μm.

8. The battery cell according to claim 7, characterized in that: The base film includes a laminate of one or more of polyethylene, polypropylene, polyimide, polyethylene terephthalate, 1313 aramid, and 1414 aramid.

9. The battery cell according to claim 1, characterized in that: The negative electrode sheet includes a negative electrode current collector and a negative electrode active layer provided on at least one surface of the negative electrode current collector in a thickness direction, wherein the negative electrode active layer includes a negative electrode active material, a conductive agent and a second binder; The negative electrode active material includes at least one of natural graphite, artificial graphite, silicon carbon, and silicon oxide; And / or, based on the mass of the negative electrode active layer, the content of the negative electrode active material is ≥92%.

10. A lithium ion secondary battery, characterized in that: The battery cell comprises the battery cell according to any one of claims 1 to 9.

Images